C1083: Cannot open include file: 'QApplication'

[Baso]

It worked, The error was that I used a QCoreApplication object instead of QApplication , my bad sorry

[china_cao]

QT +=widgets is one of assignment to qmake variable. qmake has following variables for your reference.
CONFIG
DEFINES
DEF_FILE
DEPENDPATH
DEPLOYMENT
DEPLOYMENT_PLUGIN
DESTDIR
DESTDIR_TARGET
DLLDESTDIR
DISTFILES
FORMS
GUID
HEADERS
ICON
INCLUDEPATH
INSTALLS
LEXIMPLS
LEXOBJECTS
LEXSOURCES
LIBS
LITERAL_HASH
MAKEFILE
MAKEFILE_GENERATOR
MOC_DIR
OBJECTS
OBJECTS_DIR
OBJMOC
POST_TARGETDEPS
PRE_TARGETDEPS
PRECOMPILED_HEADER
PWD
OUT_PWD
QMAKE
QMAKESPEC
QMAKE_APP_FLAG
QMAKE_APP_OR_DLL
QMAKE_AR_CMD
QMAKE_BUNDLE_DATA
QMAKE_BUNDLE_EXTENSION
QMAKE_CC
QMAKE_CFLAGS_DEBUG
QMAKE_CFLAGS_RELEASE
QMAKE_CFLAGS_SHLIB
QMAKE_CFLAGS_THREAD
QMAKE_CFLAGS_WARN_OFF
QMAKE_CFLAGS_WARN_ON
QMAKE_CLEAN
QMAKE_CXX
QMAKE_CXXFLAGS
QMAKE_CXXFLAGS_DEBUG
QMAKE_CXXFLAGS_RELEASE
QMAKE_CXXFLAGS_SHLIB
QMAKE_CXXFLAGS_THREAD
QMAKE_CXXFLAGS_WARN_OFF
QMAKE_CXXFLAGS_WARN_ON
QMAKE_DISTCLEAN
QMAKE_EXTENSION_SHLIB
QMAKE_EXT_MOC
QMAKE_EXT_UI
QMAKE_EXT_PRL
QMAKE_EXT_LEX
QMAKE_EXT_YACC
QMAKE_EXT_OBJ
QMAKE_EXT_CPP
QMAKE_EXT_H
QMAKE_EXTRA_COMPILERS
QMAKE_EXTRA_TARGETS
QMAKE_FAILED_REQUIREMENTS
QMAKE_FRAMEWORK_BUNDLE_NAME
QMAKE_FRAMEWORK_VERSION
QMAKE_INCDIR
QMAKE_INCDIR_EGL
QMAKE_INCDIR_OPENGL
QMAKE_INCDIR_OPENGL_ES1, QMAKE_INCDIR_OPENGL_ES2
QMAKE_INCDIR_OPENVG
QMAKE_INCDIR_QT
QMAKE_INCDIR_THREAD
QMAKE_INCDIR_X11
QMAKE_INFO_PLIST
QMAKE_LFLAGS
QMAKE_LFLAGS_CONSOLE
QMAKE_LFLAGS_DEBUG
QMAKE_LFLAGS_PLUGIN
QMAKE_LFLAGS_RPATH
QMAKE_LFLAGS_QT_DLL
QMAKE_LFLAGS_RELEASE
QMAKE_LFLAGS_SHAPP
QMAKE_LFLAGS_SHLIB
QMAKE_LFLAGS_SONAME
QMAKE_LFLAGS_THREAD
QMAKE_LFLAGS_WINDOWS
QMAKE_LIBDIR
QMAKE_LIBDIR_FLAGS
QMAKE_LIBDIR_EGL
QMAKE_LIBDIR_OPENGL
QMAKE_LIBDIR_OPENVG
QMAKE_LIBDIR_QT
QMAKE_LIBDIR_X11
QMAKE_LIBS
QMAKE_LIBS_EGL
QMAKE_LIBS_OPENGL
QMAKE_LIBS_OPENGL_QT
QMAKE_LIBS_OPENGL_ES1, QMAKE_LIBS_OPENGL_ES2
QMAKE_LIBS_OPENVG
QMAKE_LIBS_QT
QMAKE_LIBS_QT_DLL
QMAKE_LIBS_QT_OPENGL
QMAKE_LIBS_QT_THREAD
QMAKE_LIBS_RT
QMAKE_LIBS_RTMT
QMAKE_LIBS_THREAD
QMAKE_LIBS_WINDOWS
QMAKE_LIBS_X11
QMAKE_LIB_FLAG
QMAKE_LINK_SHLIB_CMD
QMAKE_LN_SHLIB
QMAKE_POST_LINK
QMAKE_PRE_LINK
QMAKE_PROJECT_NAME
QMAKE_MAC_SDK
QMAKE_MACOSX_DEPLOYMENT_TARGET
QMAKE_MAKEFILE
QMAKE_MOC_SRC
QMAKE_QMAKE
QMAKE_QT_DLL
QMAKE_RESOURCE_FLAGS
QMAKE_RPATH
QMAKE_RPATHDIR
QMAKE_RUN_CC
QMAKE_RUN_CC_IMP
QMAKE_RUN_CXX
QMAKE_RUN_CXX_IMP
QMAKE_TARGET
QMAKE_UIC
QT
QTPLUGIN
QT_VERSION
QT_MAJOR_VERSION
QT_MINOR_VERSION
QT_PATCH_VERSION
RC_FILE
RC_INCLUDEPATH
RCC_DIR
REQUIRES
RESOURCES
RES_FILE
SIGNATURE_FILE
SOURCES
SRCMOC
SUBDIRS
TARGET
TARGET_EXT
TARGET_x
TARGET_x.y.z
TEMPLATE
TRANSLATIONS
UICIMPLS
UICOBJECTS
UI_DIR
UI_HEADERS_DIR
UI_SOURCES_DIR
VERSION
VER_MAJ
VER_MIN
VER_PAT
VPATH
YACCIMPLS
YACCOBJECTS
YACCSOURCES
_PRO_FILE_
_PRO_FILE_PWD_